Hi,
I even work in the industry and was suprised to hear the same thing recently.
Norwich Union are one the only companies around to do this, but they are correct. Basically they will go from the first date you obtained your provisional licence, this in turn determines you in their eyes as a 'better risk' as you took longer to pass your test. I assume they are on the understanding that someone who practices on a provisional for longer (over a couple of years) is generally at a better standard of driving rather than someone who gets a provisional, takes intensive lessons for 4 months and passes first time.
So yes they have got in correct and i assume your premium cannot be matched elsewhere. Norwich Union are unique in this method and in my opinion is an all round sound product. They even let you pay over 12 equal payments.
In short, go for it, they have got everything correct and they have confirmed this over the phone where telephone calls are recorded, so even if they may have got it wrong (which i very much doubt) you have them over a barrel.